This invention relates to a control system for a direct-coupling mechanism in a hydraulic power transmission means of a transmission for automotive vehicles, and more particularly to a control system of this kind which is adapted to accurately control the engaging force of the direct-coupling mechanism so as to restrain vibrations of the vehicle body due to rotation of the engine and improve the fuel consumption of the engine.
In hydraulic power transmission means as represented by a hydraulic torque converter, a direct-coupling clutch is conventionally well known which is adapted to mechanically directly couple the input member and output member of the torque converter to enhance the transmission efficiency when the torque amplification performed by the torque converter is almost not available. This mechanical direct-coupling can produce good results advantageous in improving the power transmission characteristics, curtailing the fuel consumption, and reducing noise caused by vibration of the vehicle body. To this end, the minimum vehicle speed at which the mechanical direct-coupling is to be effected should desirably be set to a possible lowest value. However, if the mechanical direct-coupling is effected in a low vehicle speed region where also the engine speed is low, it can easily cause large vibrations of the vehicle body as well large vibration noise due to fluctuations of the engine torque which are particularly conspicuous in the low engine speed region.
In order to overcome the above disadvantage, it has been proposed by the present applicants to control the engaging force of the direct-coupling mechanism to vary in proportion to the vehicle speed or in proportion to the throttle valve opening so as to allow slippage in the direct-coupling mechanism instead of fully directly coupling same when there occur certain peak torque fluctuations during operation of the engine in the low vehicle speed region. However, in incorporating the above proposed method into an actual system, other factors should also be taken into account, that also determine the engaging force of the direct-coupling mechanism. If the engaging force is set in consideration of the extent of influence of these factors upon the engaging force as well as the margins of variation of the extent of influence, and also so as not to cause full direct-coupling of the torque converter even at the maximum extents of influence of the factors, the set engaging force has a considerably small median value, resulting in failure to attain to desired extents curtailment of the fuel consumption, improvement of the power transmission characteristics, and minimization of the vibration noise. The other factors referred to above may include, for instance, the operative state of engine load-creating devices such as an air conditioner installed in the vehicle, the atmosphere in which the torque converter is operating, and ageing changes in the operating characteristics of the torque converter, etc. Examples of the influence of such other factors upon the engaging force are as follows: When the air conditioner is operating, the engaging force is increased as the throttle valve opening then increases with an increase in the engine load caused by the operation of the air conditioner. In cold weather, the engaging force decreases while in hot weather, it increases, as the operating oil pressure in the direct-coupling mechanism is influenced by the ambient temperature. In a direct-coupling mechanism formed of a friction clutch, the friction coefficient gradually decreases from its initial or sufficient value with an increase in the accumulated time of use, to result in a gradual decrease in the engaging force.
According to these example of change of the engaging force dependent upon the factors, the engaging force is merely taken as a function of the operating oil pressure and accordingly the operating oil pressure alone is regulated to control the engaging force. However, the problem of improper engaging force can be solved by detecting the rotational speed ratio between the input and output members of the torque converter, the slip rate (=1- e) thereof, or a like factor, and controlling such a factor so as to obtain proper engaging force. Basic ideas of feedback control of the slip rate are disclosed in U.S. Pat. Nos. 3,696,896 and 3,966,032. These disclosed ideas appear to be theoretically right, and the disclosed methods employ analogue control to thereby perform smooth direct-coupling control and accordingly good driving feeling. However, according to these prior art methods, the direct-coupling mechanism is allowed to have a certain slip rate even at high speed operation when there is no fear of occurrence of vibrations of the vehicle body, thus being disadvantageous in respect of fuel consumption of the engine and effective life of the direct-coupling mechanism. Further, accurate control of the engaging force is not necessary in a low vehicle speed region where the direct-coupling mechanism is required to exhibit torque amplifying function. For example, in the aforesaid method proposed by the present applicants the engaging force of the direct-coupling mechanism is increased in proportion to the vehicle speed or to the throttle valve opening. Thus, slip should necessarily occur in the direct-coupling mechanism at first speed (low gear) operation and at second speed (second gear) operation, which operations are often applied for acceleration. Therefore, the slip rate control is not necessary to the present applicants' proposed method during these operations. Moreover, generally, at first speed operation and at second speed operation the engine rotational speed is high for the small gear ratios, with a very small possibility of occurrence of vehicle body vibrations.
Therefore, only at fourth speed (top gear) operation, and if perfect safety is desired, also at third speed (third gear) operation, the direct-coupling control system is required to effect slip rate control, while at operation such as first speed operation and second speed operation wherein slip should occur in the direct-coupling mechanism in response to peak fluctuations of the torque, no slip rate control should be carried out in view of curtailment of the fuel consumption. Further, when acceleration such as standing start of the vehicle is required, it is more advantageous in ensuring smooth driveability to allow slippage in the fluid coupling, instead of resorting to the slip rate control.
In carrying out slip rate control only at fourth speed operation, and if required, at third speed operation, to discriminate whether the vehicle is running in the fourth speed or third speed position or in another speed position is very difficult, except for a control system employing electronic control for gear shifting. For such discrimination, a special auxiliary device is required, such as a pressure switch operable to open or close at a specific operating oil pressure for engaging the fourth speed clutch or the third speed clutch, which will complicate the structure of the control system.